1. Field
The present disclosure generally relates to composite structures, and deals more particularly with a flexible truss frame for stiffening structures, as well as a method of making the frame.
2. Background
Structures such as airframes typically contain a number of lateral stiffening elements such as frames and ribs at regular intervals along the length of the structure. Conventional stiffening elements may have characteristics which increase their cost and complexity in some applications. For example, each of the lateral stiffening elements in an airframe may require a unique inner mold line surface contour in order to conform to a skin at each location along the airframe. Thus, each of the stiffening elements is a unique part.
Rigid stiffening elements often require shimming during the assembly process to accommodate buildup of manufactured tolerances. Shimming is time consuming and may require complex geometric shim shapes which may affect joint strength, stiffness and/or durability. The design of lateral stiffening elements in airframes may also be complicated by the need to span intersecting stiffeners such as longitudinal hat stiffeners and blade stiffeners. In order to accommodate these longitudinal stiffeners, the lateral frames may be provided with openings referred to as “mouseholes” next to the skin to allow passage of the longitudinal stiffeners through the frames. Mouseholes add complexity to the manufacturing process and may result in undesirable stress concentrations in the airframe.
Accordingly, there is a need for lateral stiffening elements such as frames that have a common design but which accommodate variations in surface contours at their points of attachment, thereby reducing recurring and nonrecurring costs. There is also a need for lateral stiffening elements that reduce or eliminate the need for shimming during the installation process and which have standard openings that accommodate intersecting longitudinal stiffeners.